In some circumstances, it can be desirable to leach mineral values from subterranean formation without conducting any mining operations. Such a technique can be useful, for example, where the grade or location of the ore body or geologic conditions make extraction of the mineral values by conventional mining uneconomical. In such a situation, it can be desirable to leach the mineral values directly from the in situ subterranean formation.
In an exemplary in situ leaching operation, two or more wells are drilled to the portion of the subterranean formation containing desired mineral values. A leachant or lixiviant can be introduced in one or more wells to permeate the subterranean formation and a pregnant solution containing dissolved mineral values can be withdrawn from one or more adjacent wells.
For example, a plurality of wells can be drilled several hundred feet to an ore body containing copper minerals to be leached. These can be oxidized copper minerals which are considered "soluble" since they can be dissolved in sulfuric acid solution, or "insoluble" copper minerals such as sulfides which require oxidation before they can be dissolved. A sulfuric acid solution, which can also contain ferric sulfate or the like for oxidizing insoluble copper minerals, is introduced as a leachant through one or more such wells. Sufficient pressure is maintained in the inlet well that the leach liquid permeates through the copper ore body to one or more outlet wells. A pregnant solution containing copper values leached from the subterranean formation is withdrawn from the outlet well. The leach solution can flow through inherent permeability in the formation or at least in part through artificially induced permeability. Such artificial permeability can be produced in the subterranean formation by hydraulic fracturing, for example.
The inherent permeability in many subterranean formations can be rather low and to obtain a reasonable volume of flow of liquid through the formation, rather high pressure gradients can be employed between inlet and outlet wells. An inlet well can, for example, have a wellhead pressure of several hundred psi. This pressure is superimposed on the hydrostatic head of the column of liquid in the well. In an embodiment where flow from an outlet well is induced by pressure applied to an inlet well, the outlet well has the hydrostatic head of pregnant solution in the well. Alternatively, the pressure in an outlet well can be substantially less than the hydrostatic head where a submersible pump or air lift is employed for withdrawing pregnant solution.
After a well for in situ leaching has been drilled, it may be completed with fiber reinforced plastic pipe as a well bore casing. Such glass fiber reinforced plastic pipe is inserted in the well bore and the annulus between the pipe and surrounding formation is closed by pumping a grout of Portland cement or the like around the casing. This provides a casing in the well bore which is resistant to leach solutions such as sulfuric acid.
It is desirable to localize the introduction or withdrawal of liquid between the well bore and surrounding formation. The casing is, therefore, perforated at a desired elevation in the well. Standard perforating tools employing projectiles, explosive charges, cutters, or the like, as commonly employed in oil wells are used for perforating the casing in a well for in situ leaching. If desired, hydraulic fractures can be induced adjacent such perforations. Such fracturing is induced after isolating a section of the well bore at the elevation where fracturing is desired. This section can be isolated by a conventional packer when near the bottom of a well or by a conventional straddle packer when at an elevation remote from the bottom of the well. Sufficient hydraulic pressure is applied in the isolated section of the well to induce fracturing of formation adjacent the well.
When a well is used for introducing leach liquid, it can be desirable to insert tubing down the casing and set a packer in the casing at an elevation above the perforations through the casing through which it is desired to introduce leach liquid. Leach liquid is then introduced through such tubing so that hydraulic pressure is applied only in the portion of the casing below the packer. This avoids subjecting the entire casing to the elevated pressures employed for inducing flow of leach liquid through the formation.
Damage to the plastic casing can occur when various tools are run in the well and the casing can deteriorate in some situations when subjected to prolonged elevation pressure and strong leaching solutions. Such damage can result in undesired flow of leach liquid between the well bore and surrounding formation. Thus, it sometimes occurs that a path for fluid flow can occur between a portion of the casing below the packer and a damaged portion of the casing above the packer. This flow can be through a portion of the annulus around the casing where the cementing is inadequate or can be through permeability in the formation surrounding the well bore. Damage to the casing can result in leakage of leach liquid into the annulus between the inlet tubing and the casing set in the well. Since such leakage is undesirable for a number of reasons, it is important to have means for repairing the well for stopping or limiting such flow of leach liquid.
A technique sometimes employed for repair of well bores is known as a cement squeeze. In such a technique, a portion of a well bore to be repaired is isolated by a straddle packer or the like. A grout of Portland cement, pozzolana cement, or the like is introduced into the isolated section of the well bore and sufficient pressure is applied to squeeze grout from the casing into porous regions outside the casing. Excess grout can be flushed from the well bore or if it should set in the well bore, the hardened cement can be drilled out.
A cement squeeze job ordinarily calls for brining a service company to the well to perform the cementing. Cement squeezes are required only occasionally and the service companies provide the specialized equipment and expertise for performing this job. Appreciable time can be lost waiting for a service company to respond when a request is made for a well repair. Such service companies are primarily located in areas where there is petroleum production and in situ leaching operations can be rather remote from such locations. Thus, distance as well as scheduling of the service company can cause appreciable time delays. The additional travel distance can also add to the already expensive costs involved in making a cement squeeze.
It is, therefore, desirable to provide a technique for repairing a well bore for in situ leaching operations which can be done quickly and with equipment ordinarily at the in situ leaching site. The technique should be inexpensive, fast, and readily accomplished by field personnel.